In measuring medical samples a fundamental distinction is made between one-way sensors and flow measuring cells. In the case of a one-way sensor the sample is introduced into the sensor and brought into contact with measuring electrodes. The basic requirement for an accurate and error-free measurement is a suitable positioning of the sample in the measuring cell. It is a known procedure to check the positioning via special measuring contacts, to which an AC voltage is applied, such that an impedance measurement will produce a signal which provides information regarding the position of the sample. Due to the distance between the electrodes for the measurement proper and the electrodes for position-checking, errors in the measurement result may occur.
From WO 99/32881 a one-way measuring cell is known which avoids the above disadvantage by applying an alternating voltage to the measuring electrodes themselves. It is possible in this way to check the exact positioning of the sample as a first step and then to proceed to the measurement itself or to reject the sample if the positioning is found to be at fault. Furthermore, flow cells with a multitude of electrode systems are unknown, which are suitable for a series of measurements or for continuous measurement and which determine the concentration of diverse analytes in a sample. Conditions in flow cells of this sort differ fundamentally from those in one-way cells. It is for instance not sufficient in this type of flow cells to check the positioning of the sample prior to measuring, as it will of course change during the measurement process. A further problem occurs if electrochemical reactions due to the measurement voltage cause bubble formation at an electrode, which is undesirable and may result in measurement errors.